1. Field of the Invention
The present invention relates to a high pressure discharge lamp and, more particularly, to improvements in an electrode used in a high pressure discharge lamp.
2. Description of the Prior Art
High pressure discharge lamps, for example mercury lamps, xenon lamps, and the like, typically include pair of elengated electrodes consisted consisting of very high melting point metal such as tungsten. Each electrode of a pair has a discharge end extending in a discharge space defined by a hermetically sealed enclosure made of hard glass such as quartz glass or bolosilicate glass and facing the other electrode. The electrodes also have connection ends embedded in holding portions of the glass enclosure extending outward therefrom. The connection ends of the electrodes are electrically led out the glass enclosure through metal lead foils consisting of high melting point metal such as molybdenum.
The discharge ends of the electrodes are made thick to ensure heat resistance. In particular, the discharge ends of the pair of electrodes of AC (alternating current) type high pressure discharge lamps, and one electrodes of DC (direct current) type high pressure discharge lamps, are made extremely thick. In the DC type discharge lamps, the pair of electrodes form respectively cathodes and anodes, as is well understood in the art. For simplicity, the anodes and the pair of electrodes are referred hereinafter simply as electrode. On the other hand, the connection ends of the electrodes have been restricted in size in their transverse direction. This is done in order to thicken the holding portions of the glass enclosure so that their pressure resistance is increased, according to the size restriction of the connection ends. Thus, the connection ends of the electrodes are reduced in diameter relative to the discharge ends, or flattened to a plate shape with reduced thickness.
The metal lead foils must be increased in size to increase their current-carrying capacity. However, the thickness of the metal lead foils is restricted to about 20 .mu.m or at most 35 .mu.m for reliable bonding to the quartz glass or the like by heat-welding.
In conventional high pressure discharge lamps, the diameter or thickness of the connection ends has been uniform along their elongated direction over their whole length. When high pressure discharge lamps are repeatedly switched on, the connection ends of the electrodes and the metal lead foils repeatedly thermally expand to a considerable extent. As a result, the metal lead foils are easily damaged by the repeated thermal expansion.
Further, the connection ends have tended to become loose in the holding portions as a result of repeated thermal expansion, so that the electrodes are no longer securely held in the glass enclosure.
In addition, the connection ends of the electrodes are repeatedly pressed against the holding portions of the glass enclosure by the thermal expansion. Particularly in the case of flattened connection ends, the corners of the connection ends along the elongated direction exert great pressure on the holding portions. If the thickness of the flattened connection ends is small, the pressures against the holding portions are concentrated on a small region. As a result the holding portions of the glass enclosure are easily damaged or cracked by accumulation of stresses due to the excessive stress concentration.